This invention relates generally to oscillator circuits, and, in particular, to a crystal oscillator in which the duty cycle may be varied.
A wide variety of oscillator circuits is known in the electronic arts. See, for example, Integrated Electronics: Analog and Digital Circuits and Systems, J. Millman and C. C. Halkias, McGraw-Hill Book Company, New York, 1972, Chapter 14. One such type of oscillator circuit comprises an active device, such as a bipolar transistor, op amp, or FET, and a voltage-series feedback loop, in which the load impedance Z.sub.L consists of an inductance L in parallel with a series combination of two capacitances C.sub.1 and C.sub.2. This circuit arrangement is known generally as a Colpitts oscillator. A typical Colpitts configuration is shown at page 492 of the above-cited reference.
It is also well known to employ a piezoelectric crystal in an oscillator circuit to achieve high frequency stability with regard to time and temperature. Various crystal oscillator circuits are described beginning on page 495 of the above-cited reference.
In the field of television circuits and associated equipment for generating video signals suitable for utilization with television circuits, it is sometimes desirable to provide a clock oscillator circuit with means for varying the duty cycle thereof. The duty cycle of an oscillator may be defined as the ratio of the width of the positive-going portion of its output signal with respect to the combined widths of the positive-going and the negative-going portion during one cycle of such signal expressed as a percent.
A known oscillator circuit having an adjustable duty cycle is disclosed in U.S. Pat. No. 3,812,387, assigned to the assignee of the present invention. In the aforementioned U.S. patent, which is directed to a horizontal processing circuit for a television receiver, a sawtooth signal is generated by a horizontal oscillator and is used to control a semiconductor switch to which a reference voltage is applied. The semiconductor switch conducts when the applied sawtooth signal is below the reference voltage level, and the duty cycle of the rectangular-wave signal output may be varied by changing the reference voltage level. While the operation of the aforementioned variable duty cycle oscillator is generally satisfactory for its intended purpose, it is desirable to have a variable duty cycle oscillator which generates a high frequency output signal that is very stable.